The present invention relates to AC power control, and more particularly to duty cycle based AC power control with reduced voltage fluctuations in order to satisfy electrical equipment standards and eliminate noticeable "flicker" in other appliances when the electrical equipment imposes voltage fluctuations upon an AC power line.
IEC Standard 1000-3-3 specifies certain tests that electrical equipment connected to an AC line must pass in order to be sold in Europe. In general terms the Standard describes how voltage fluctuations imposed on the AC line by an appliance, due to time-varying power demand, must be limited so as to reduce effects on other devices connected to the line. The canonical example is the visible flickering of incandescent lamps caused by these voltage fluctuations. The Standard defines numerical methods to quantify this flickering, prescribes conditions under which it must be measured, and sets allowable limits for it.
Certain devices, such as phase change ink printers, require various internal subsystems to be maintained at specific values, such as for temperature control. This is done using closed-loop regulation with "continuously variable" load actuation. The continuously variable actuation is simulated by cycle dropping, for example, by connecting loads to the line for full cycles starting at line zero crossings. Cycle dropping also is used to compensate for line voltage so that a given load control loop's output signal values result in the same average power input to the load over the entire line voltage range. For example a load that draws three amperes at one hundred volts to produce 300 watts of power draws six amperes at two hundred volts to produce 1200 watts of power, so the load only needs to be activated one-quarter of the time for the same average power, which reduces the duty cycle in this case to 25% or one out of every four cycles. Also where the loads are of varying sizes and are independently actuated, the active (on) cycles of various heaters occur at random times with respect to each other and the power gets spread across the frequency spectrum like "white" noise. The result is that for multiple loads with multiple demands there can be an appreciable amount of power at the most sensitive "flicker" frequency of 8.8 Hz.
What is desired is a duty cycle based AC power control method with reduced voltage fluctuations that minimizes "flicker" in other devices coupled to the same line voltage.